In recent years, as the resolutions of image display devices such as LCDs (liquid crystal displays) are enhanced, processing for enlarging digital images becomes important.
FIG. 1 shows a configuration example when an enlarging device using nearest-neighbor interpolation is applied to a conventional OSD (on-screen display) device 1.
A microcomputer 11 in the OSD device 1 controls storage of a font ROM (read only memory) 12 and an OSD-plane memory 13 in accordance with a signal (hereinafter referred to as a “user operation signal”) that is sent from a user interface 2 and that corresponds to a user operation. The font ROM 12 stores fonts, such as characters, symbols, or graphics to be displayed on a monitor 3. As fonts stored in this case, various typefaces, such as Japanese fonts (i.e., fonts for Japanese) and European fonts (i.e., fonts for alphabets), are available. The OSD-plane memory 13 stores, as OSD display data, fonts read from the font ROM 12 by the microcomputer 11. Further, under the control of the microcomputer 11, the OSD-plane memory 13 supplies the OSD display data to a 2×2 nearest-neighbor interpolator 14. The 2×2 nearest-neighbor interpolator 14 enlarges the OSD display data, supplied from the OSD-plane memory 13, by a factor of 2 in the vertical directions and a factor of 2 in the horizontal directions and supplies the resulting OSD display data to a mixer 15. The mixer 15 combines the OSD display data supplied from the 2×2 nearest-neighbor interpolator 14 and a video signal supplied from a camera processor, not shown, and causes the monitor 3 to display the resulting data.
FIG. 2 is a diagram showing a configuration example when fonts for ordinary display and for enlargement display are stored in the font ROM 12 of the conventional OSD device 1. Units corresponding to those in FIG. 1 are denoted by the same reference numerals and the descriptions thereof are omitted as appropriate.
The font ROM 12 stores fonts for ordinary display and for enlargement display, such as characters, symbols, and graphics to be displayed on the monitor 3. The OSD-plane memory 13 stores, as OSD display data, fonts read from the font ROM 12 by the microcomputer 11. Further, under the control of the microcomputer 11, the OSD-plane memory 13 supplies the OSD display data to the mixer 15. The mixer 15 combines the OSD display data supplied from the OSD-plane memory 13 and a video signal supplied from a camera processor, not shown, and causes the monitor 3 to display the resulting data.
As described above, with the OSD device 1 shown in FIG. 1, although the cost is low, the image quality deteriorates significantly. With the OSD device shown in FIG. 2, although the image quality can be maintained, a large-capacity font ROM 12 and a large-capacity OSD-plane memory 13 are required.
Digitized image can be broadly classified into two groups, namely, natural images captured by image-capture devices, such as digital still cameras, and artificial images, such as graphics and characters generated by computers and so on.
For natural images, interpolation-filter-based enlargement techniques and reduction techniques according to sampling theorems, such as bilinear interpolation and cubic interpolation, are used. These techniques can provide high-quality enlarged images with respect to natural images that have multi-value gradations and that contain noise in principle.
In contrast, when an interpolation-filter-based enlargement technique is used for artificial images, edges of characters and so on are rounded, thus making it impossible to provide high-quality images. In particular, for binary images such as characters and so on, blurring due to edge rounding occurs. Accordingly, in order to prevent the blurring, an enlargement technique using nearest-neighbor interpolation, as shown in FIG. 1, is used for artificial images. With this technique, however, jaggy becomes a problem in a visual sense.
Accordingly, some techniques for performing high-quality enlargement on, of artificial images, image data (e.g., characters and graphics) that has less gradation and that is less susceptible to noise have been proposed.
For example, Patent Document 1 proposes a technique for generating enlarged characters from basic-character font data and correcting the characters. In addition, for example, Patent Document 2 proposes a technique for scaling binary images by using piecewise polynomial interpolation. Additionally, for example, Patent Document 3 proposes a technique for generating fonts by using a genetic algorithm. Also, for example, Patent Document 4 proposes a technique for recognizing a coupling state of surrounding pixels, estimating an optimum contour, and performing re-sampling. In addition, for example, Patent Document 5 proposes a technique for smoothing when characters or graphics are enlarged.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 5-94171    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 8-63592    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2004-4302    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 6-68247    [Patent Document 5] Japanese Unexamined Patent Application Publication No. 9-305755